1. Field of the Invention
This invention relates in general to alignment apparatus for an electron beam in a cathode ray tube for externally adjusting the alignment of the electron beam.
2. Description of the Prior Art
Conventional alignment apparatus for electron beams of cathode ray tubes utilize two rotatable permanent magnet rings which surround the outer portion of the tube where the correction is desired. By adjusting the angular position of these rotatable magnetic rings, the magnitude and direction of the magnetic field for the electron beam in a cathode ray tube can be controlled and the alignment of the electron beam can be made. In the prior art devices, a magnetic shield plate does not generally surround the outside of the portion where the adjustment of the electron beam occurs. Thus, the electron beam is subjected to the influence of an external or environmental magnetic field. So as to avoid the influence of such external magnetic field, a magnetic shield plate may cover the outside of the two magnetic rings. However, in this structure, the means for rotating the magnetic rings relative to each other becomes very complicated.
A second conventional alignment apparatus for electron beam of the prior art comprises an alignment correction coil which surrounds the outside of the electron gun where the beam alignment correction is required. Orthogonal electromagnetic fields generated by the alignment correction coil correct the alignment of the electron beam.
A magnetic shield plate extends over the alignment correction coil to prevent the influence of external and environmental magnetic shields. In this structure, however, there must be applied a constant supply of current for generating the orthogonal electromagnetic field and the structure becomes complicated and expensive and high power is consumed on a continuous basis.